1. Technical Field
The present disclosure relates to a semiconductor integrated device assembly and to a related manufacturing process. In particular, the following description will refer, without this implying any loss of generality, to the assembly of an acoustic transducer of a MEMS (microelectromechanical system) type.
2. Description of the Related Art
As acoustic transducer, for example a MEMS microphone of a capacitive type, generally comprises a micromechanical sensing structure, designed to transduce acoustic pressure waves into an electrical quantity (in the example a capacitive variation), and a reading electronics, designed to carry out appropriate processing operations (amongst which operations of amplification and filtering) of the electrical quantity to supply an electrical output signal, for example a voltage.
In greater detail, and with reference to FIG. 1, a micromechanical sensing structure 1 of a MEMS acoustic transducer comprises a structural layer 2 of semiconductor material, for example silicon, in which a cavity 3 is obtained, for example via chemical etching from the back. A membrane 4, or diaphragm, is coupled to the structural layer 2 and closes the cavity 3 at the top; the membrane 4 is flexible and, in use, undergoes deformation as a function of the pressure of incident sound waves. A rigid plate 5 (generally known as “back-plate”) is set above the membrane 4 and facing the latter, via interposition of spacers 6 (for example, made of insulating material, such as silicon oxide). The rigid plate 5 constitutes the fixed electrode of a variable-capacitance detection capacitor, the mobile electrode of which is constituted by the membrane 4, and has a plurality of holes 7, designed to enable free circulation of air towards the same membrane 4 (making the rigid plate 5 “acoustically transparent”). The micromechanical sensing structure 1 further comprises (in a way not illustrated) membrane and rigid-plate electrical contacts, used for biasing the membrane 4 and the rigid plate 5 and collecting a capacitive variation signal as a consequence of the deformation of the membrane 4 caused by the incident acoustic pressure waves.
The sensitivity of the MEMS acoustic transducer depends on the mechanical characteristics of the membrane 4 of the micromechanical sensing structure 1 (in particular upon its so-called mechanical “compliance”), and the type of assembly of the membrane 4 and of the rigid plate 5.
In addition, the volume of the front acoustic chamber, the so-called “front chamber” (i.e., the space traversed in use by the acoustic pressure waves coming from the external environment through an appropriate access port), and the volume of the rear acoustic chamber, the so-called “back chamber” (i.e., the space that is located on the opposite side of the front chamber with respect to the membrane 4, set in use at a reference pressure) directly affect the acoustic performance of the transducer.
In particular, the volume of the front chamber determines the upper resonance frequency of the acoustic transducer, and hence its performance at high frequencies (in fact, the operating frequency band of the acoustic transducer has to be lower than the resonance frequency of the oscillations of air). In general, the smaller the volume of the front chamber, the higher the upper cut-off frequency of the transducer in so far as the resonance frequency of the oscillations of the air shifts towards higher frequencies.
The back chamber behaves, instead, as a closed volume subject to compression, with the consequence that the smaller the volume of the back chamber, the lower the sensitivity of the acoustic transducer (in fact, it is as if the deformations of the membrane were hindered by the action of a spring of high stiffness). It is hence desirable in general to provide a back chamber of large dimensions so as to improve the sensitivity of the acoustic transducer.
The volume of the front chamber and/or of the back chamber of the MEMS acoustic transducer depend not only upon the configuration of the micromechanical sensing structure 1, but also upon the conformation of the related package, i.e., the container, casing, or coating, which surrounds, totally or in part, the die or dice of semiconductor material of the acoustic transducer, enabling electrical connection thereof from outside. The package is configured to house not only the micromechanical sensing structure 1 itself, but also the reading electronics associated thereto, generally provided as an ASIC (application-specific integrated circuit), integrated in a respective die of semiconductor material.
FIG. 1 shows by way of example a package solution for the MEMS acoustic transducer, here designated as a whole by 10, housing a first die 11, integrating the microelectromechanical structure 1, and moreover a second die 12, which also includes semiconductor material, integrating an ASIC, electrically coupled to the microelectromechanical structure 1 and designated as a whole by 13.
In this solution, the first and second dice 11, 12 are coupled side by side on a substrate 14 of the package. Electrical connections 15 between the first and second dice 11, 12 are provided with the wire-bonding technique between corresponding contact pads, designated as a whole by 16, whilst appropriate metallization layers and vias (not shown in detail) are provided in the substrate 14 for routing the electrical signals towards the outside of the package. Further electrical connections 17, obtained with the wire-bonding technique, are provided between the second die 12 and a top face of the substrate 14, coupled to which are the same dice 11, 12.
A cap 18 of the package is also coupled to the substrate 14, enclosing inside it the first and second dice 11, 12. The cap 18 may be made of metal or pre-molded plastic with an internal coating metallization layer 18a so as to prevent the disturbance of external electromagnetic signals (by providing a sort of Faraday cage). The cap 18 has an opening 19 to enable introduction of an air flow from the outside and of acoustic pressure waves.
Electrical contact elements (not shown), for example, in the form of conductive lands or bumps, are provided on the bottom part of the substrate 14 for soldering and electrical connection to an external printed circuit.
There are several constraints imposed on the assembly of a MEMS acoustic transducer, which render particularly problematical the design thereof, in particular in the case where extremely compact dimensions are utilized, as, for example, in the case of portable applications.
In order to reduce the lateral encumbrance, in the U.S. patent publication No. 2013/0032936 filed in the name of the present Applicant on Jun. 30, 2011, a vertically stacked packaging structure has been proposed, represented and designated by 20 in FIG. 2, comprising a first composite substrate 21 and a second composite substrate 22, which are stacked and fixed on one another, each carrying a respective die 11, 12 of the MEMS acoustic transducer.
Each composite substrate 21, 22, shown schematically in FIG. 2, is obtained as described in FIGS. 3a-3c, using the technology described in detail in the U.S. patent publication No. 2012/0153771 filed in the name of the present Applicant.
In particular, FIG. 3a illustrates the coupling of a base layer 23 and of a wall layer 24 made of a same plastic material, in particular an epoxy resin, specifically a laminated BT (bismaleimide triazine). A first main face 23a of the base layer 23 and a first main face 24a and a second main face 24b of the wall layer 24 are coated, using standard techniques, by respective thin metal layers 25, 26, 27. In addition, an adhesion layer 28, made of non-conductive adhesive material, is formed, for example, on the metal layer 26 that coats the first main face 24a of the wall layer 24, which is to be fixed to the base layer 23.
A chamber 29 has been formed (e.g., via a conventional chemical etching or laser drilling process) throughout the thickness of the wall layer 24 so as to remove locally also the metal layers 26, 27 and the adhesion layer 28.
Next, as indicated by an arrow in FIG. 3a, the base and wall layers 23, 24 are joined together, in a stacked manner, so as to form a composite substrate, illustrated in FIG. 3b. In this way, a chamber is formed, once again designated by the number 29 for simplicity, having side walls, defined by the wall layer 24, and a bottom, defined by the base layer 23.
A seed layer 30 is made to grow on the remaining portions of the metal layer 27 on the second main face 24b of the wall layer 24, on the side walls and on the bottom of the chamber 29. Next, for example using an electroplating technique or a sputtering technique, on the seed layer 30 a second metal layer 31 is made to grow, which coats, in particular, the inside of the chamber 29, to form, together with the layers 25, 27 and 30, a coating layer 32, also coating the walls and the bottom of the chamber 29.
Then, FIG. 3c (which shows the composite substrate set upside down), using standard micromachining techniques, an opening 33 is formed through the base layer 23 and the coating layer 32.
The coating layer 32 is also suitably processed, in a region corresponding to the second main face 24b of the wall layer 24, in such a way as to define electrical-contact structures, and in particular contact pads 34 and, more externally, a frame-shape contact region 35 (designed to enable provision of an electromagnetic shield, when connected to an appropriate reference potential).
As shown in FIG. 2, the composite substrates 21, 22 are stacked vertically by interposition of an adhesive layer 36, and have a different extension of the respective chamber 29, so that the respective dice 11, 12, housed in the chamber, are staggered laterally and in part vertically set one above the other. In particular, the first composite substrate 21 houses, within the respective chamber 29, the first die 11 integrating the microelectromechanical structure 1 (here not shown), and the second composite substrate 22 houses, within the respective chamber 29, the second die 12 integrating the ASIC 13 (here not shown).
The access opening 33 of the first composite substrate 21 enables access of the acoustic waves into the stacked package structure, whereas the respective access opening 33 of the second composite substrate 22 sets the respective chambers 29 in communication, and in particular enables electrical connection between the dice 11, 12 through the electrical connections 15. The further electrical connections 17 connect the second die 12 to the contact pads 34 carried by the side wall of the second composite substrate 22.
In a way not illustrated in FIG. 2, a base substrate is also set on top of the second composite substrate 22 (with a function similar to that of substrate 14 of FIG. 1), through which electrical connections are provided between the contact pads 34 and the outside of the package structure.
The package structure described has the advantage of reducing the overall dimensions of the MEMS acoustic transducer, thanks to the vertical stacking of the corresponding dice.